In nuclear power plants all components that come into contact with radioactive substances are contained in the so-called “nuclear island”. This comprises the safety container (inner containment) with primary circuit, the flood basin and the core catcher. In the top part of the safety container catalytic recombiners or recombiner systems can be installed, for example with Pd on Al2O3, which are meant to limit the proportion of hydrogen in the atmosphere, in order to prevent hydrogen explosions.
In fission reactors, during normal operation, but in particular in the event of a failure, in addition to the solid decay isotopes of uranium, or daughter isotopes arising from the uranium fission and higher isotopes resulting from neutron capture, gaseous radioactive compounds also occur, which must on no account be released into the environment. The most important of these, in addition to hydrogen, are radioactive iodine and methyl iodide.
Hydrogen is produced in small quantities in normal operation and in large quantities in the event of failures which are associated with a significant increase in temperature due to the reaction of water with the metal casing of the fuel rods. This hydrogen can then, in a detonating gas explosion, lead to the destruction of the safety container and to the release of large quantities of radioactive material (e.g. the accidents at Chernobyl and Fukushima). In order to avoid such disasters, more recently nuclear power plants are being equipped or retrofitted with recombiner systems. These are passive systems, the object of which is to catalytically reoxidize hydrogen formed at room temperature under atmospheric conditions, to form water vapour and thus avoid the production of explosive atmospheres. Recombiners can also be used for spent fuel pools and fuel element containers, ensuring that the hydrogen released can react to form water before it reaches an explosive concentration.
From the fission products caesium and iodine, caesium iodide is also produced in the fuel rods, which, unless it is retained in the fuel rods, collects in the reactor sump. Due to radiolysis or at hot spots (e.g. at hot spots in hydrogen recombiners) elemental iodine is formed from caesium iodide in the reactor sump, and can escape from the reactor sump due to its volatility, even in normal operation. Due to its reactivity with organic substances from the reactor environment (e.g. dye), free iodine can then react to form methyl iodide. These volatile radioactive substances accumulate in the gas stock inside the reactor shell and have to be adsorbed from there. Many of the compounds formed are also present aerosol-bound and are released in the event of a failure.
A majority of the radioactive iodine isotopes formed have a short half-life and thus, due to the high radiological activity in the event of a failure, contribute very significantly to the danger to life. Iodine is taken up in the thyroid gland and, at high concentrations, causes thyroid cancer. In particular iodine 131 with a half-life of 8 days may be mentioned here.
There is therefore a need for radioactive methyl iodide adsorbers that are stable even under high humidity, for equipping nuclear reactors or for retrofitting in safe operation or to ensure the safety of the reactor when shut down or during or after decommissioning.
Based on new knowledge and safety conditions, the retention of organic methyl iodide has now become a challenge, which is also a lesson learnt from the Fukushima incident of 2011. Until now there has been no technical solution. However, various approaches have been pursued. Adsorbent materials for the deposition of elemental iodine are known, such as e.g. aluminium oxides loaded with silver. However, due to inhibition, these are not very useful under water vapour.
Until now there has been no operational technically sophisticated measure for depositing methyl iodide. It is known that silver-containing adsorbent materials are suitable for the adsorption of methyl iodide. The silver iodide formed has a melting point of ca. 600° C. and a boiling point of ca. 1500° C., and is therefore largely stable under normal conditions. However, in the case of known adsorbent materials, under high water vapour concentrations, it is possible for water to be incorporated in the porous structures of the adsorber and thus inhibit the adsorption of methyl iodide. A possibility for preventing this inhibition is silanization of the outer adsorber surface by means of organic silane compounds. This method is expensive and presents serious technical problems. The silane layer decomposes as from ca. 180° C., generating considerable heat. This means that, starting from this temperature no further adsorption of methyl iodide can take place and furthermore, any hydrogen that may be present will possibly be ignited due to the highly exothermic conditions, which may lead to the undesired concomitant phenomenon of an explosion.